A typical example of a known method of transmitting data items as mentioned in the opening paragraph is disclosed in "Protocol for a TV-Guide using Electronic Data transmission, Draft 1", EACEM Technical Report No. 11, August 1995. In this prior-art method, each data item accommodates programming information such as start time, stop time, title, etc. of a television program to be broadcast. Each data item includes an identifier (a 24-bit field denoted program.sub.-- id) for identifying each programming data item. Also transmitted is overhead or system information which includes, inter alia, a field (no.sub.-- of.sub.-- programmes) indicating how many data items are being delivered by the information provider.
A possible carrier for the transmission of electronic programme guide (EPG for short) data items are the picture lines in the vertical blanking interval (VBI) of a television signal. The data items are transmitted cyclically so as to allow television receivers and video recorders to collect and store the items on a regular basis and keep the stored information updated.
As the transmission capacity of the carrier is generally restricted (the VBI is also used for test signals and teletext) and the amount of data to be transmitted may be very large, it is envisaged to transmit some data items more frequently than others. For example, the programming data for television programmes to be broadcast within the next few hours or so will be transmitted once per few minutes, whereas the programming data for programmes to be broadcast the following day or days will be transmitted once per few hours or even once per day.
It has been proposed to number the data items consecutively, by virtue of their identifier, in a predetermined order. In the EPG application, for example, the data items are envisaged to be numbered in the order of the scheduled start time of the relevant television programme. If K data items are transmitted in total, the data item representing the next programme to be broadcast will thus be assigned the number 0, whereas the data item representing the last programme to be broadcast next will be assigned the number K-1. The proposed strategy renders it possible for a receiver to check whether all data items have been received, and to detect which data item(s) is (are) missing.
FIG. 1 shows a timing diagram to illustrate the disadvantages of this strategy. The Figure shows a data stream comprising a plurality of data items Di, the index i being a number assigned to the relevant data item by virtue of its identifier. As has been attempted to show, the data item D0 is transmitted more frequently than the data items D1 . . . D4. The data stream further includes the regular transmission of a field F=k, the value k representing the number of transmitted data items. As the Figure illustrates, five data items D0 . . . D4 are transmitted up to t=t.sub.1. Thereafter, data item D0 is no longer transmitted because, for example, the relevant television programme has already been broadcast. The above-mentioned strategy of transmitting a field indicating the number of data items, and consecutively numbering the data items, requires the transmitter to decrease the field value k by 1 and to renumber the respective identifiers i from 1 . . . 4 into 0 . . . 3 after t=t.sub.1. In other words, data item D0 transmitted after t=t.sub.1 is not the same as data item D0 transmitted before, but corresponds to former data item D1.
It is cumbersome for a transmitter to renumber all the data items for transmission, i.e. to update all the identifiers. Moreover, it is difficult for a receiver to process the updated field value k and identifiers i, and check the correctness and completeness of the received information. For example, from t=t.sub.1 the receiver has been informed by virtue of the field F=4 that there are four relevant data items to be acquired. They are necessarily numbered D0 . . . D3. However, at t=t.sub.2, the data items D0 (former D1) and D1 (former D2) have been updated, whereas the former data items D2 . . . D4 are still locally stored. Not only does the number of stored data items correspond any longer to the number as transmitted, the receiver has also stored one data item twice in the memory with two different identifiers (new D1 and former D2). Needless to say that this strategy is a potential source of problems and, consequently, processing errors.